The present invention relates to a finder optical system of a single-lens reflex camera, and more specifically, to a finder optical system that is suitable for a medium-format camera to which a film back is attached.
In general, a single-lens reflex (SLR) camera for a 35 mm film employs a pentaprism finder that has a pentaprism as an erecting optical system. A finder window, which is the last optical surface of the finder optical system, is positioned to be coincident with a back surface of a camera body in the finder of the SLR camera for a 35 mm film.
On the other hand, the pentaprism finder may not be suitable for a medium-format SLR camera that uses a Brownie film whose width is 61.5 mm. When a medium-format SLR camera employs an exchangeable film back, the film back is jutted over the back surface of the camera body. If such a camera is provided with the pentaprism finder, an eye relief becomes too large to observe an object through the finder.
U.S. Pat. No 4,171,888 discloses a finder optical system that consists of a relay optical system employing a trapezoidal prism, and a magnifying optical system. The relay optical system forms a second image by erecting an inverted first image formed on a focusing screen (a first image plane) through a taking lens of a camera. The second image is observed through the magnifying optical system under magnification.
In general, a ratio of a viewing area of a finder to an image area on a film, which is referred to as a finder viewing ratio, and a finder magnification, which represents a size of an observed image, are important factors to design the finder optical system. It is preferable that the finder viewing ratio and the finder magnification are large as possible. However, it is difficult to increase the finder viewing ratio and the finder magnification in balance for the finder employing a trapezoidal prism. That is, when sizes of incident and exit surfaces of the trapezoidal prism are designed to be large in order to increase the finder viewing ratio, the total optical path length in the prism becomes too large to keep the appropriate finder magnification.
It is therefore an object of the present invention to provide an improved finder optical system that is capable of increasing the finder viewing ratio and the finder magnification in balance, assuming that the finder optical system includes a relay optical system that forms a second image by erecting an inverted first image formed on the first image plane and a magnifying optical system through which the second image is observed under magnification.
For the above object, according to the invention, there is provided a finder optical system that is provided with a relay optical system that forms a second image by erecting an inverted first image formed on a first image plane through a taking lens of a camera, and a magnifying optical system through which the second image is observed under magnification. The relay optical system includes a condenser lens, a reflecting prism and a relay lens arranged in the order from the first image plane toward an eyepoint. The following conditions (1) and (2) are satisfied;
(1) 0.25 less than |mr| less than 0.35
(2) 0.75 less than f2/f3 less than 0.85
where mr is magnification of the relay optical system, f2 is a focal length of the relay lens, and f3 is a focal length of the magnifying optical system.
The relay lens may include at least one positive lens and at least one negative lens that are arranged in the order from the second image plane toward the taking lens, and wherein the following condition (3) is satisfied;
(3) 1.3 less than |f2/f2n| less than 2.5.
The symbol f2n is a focal length of the negative lens that is the closest to the second image plane in the relay lens. When the closest negative lens consists of a plurality of negative lens elements, the symbol f2n represents a resultant focal length of the plurality of negative lens elements.
The magnifying optical system may include at least one positive lens and at least one negative lens that are arranged in the order from the second image plans toward an eyepoint, and wherein the following condition (4) is satisfied;
(4) 0.7 less than |f3/f3n| less than 1.5.
The symbol f3n is a focal length of the negative lens that is the closest to the second image plane in the magnifying optical system. When the closest negative lens consists of a plurality of negative lens elements, the symbol f3n represents a resultant focal length of the plurality of negative lens elements.
Further, the reflecting prism may be a trapezoidal prism.